When you saw "Star Wars: The Force Awakens" for the fourth time in theaters, did you see any niggling issues with gravity? Or the fact that alien worlds had breathable atmospheres? The characters were conveniently zipping from planet to planet and walking on each surface with no apparent problem.

Is this actually true for most exoplanets we know of? After all, several of the planets in our solar system aren't that friendly for a stroll. Jupiter's pressure would crush you long before you reached the surface - if there is a "surface" buried beneath the gas layers. Venus-landing spacecraft had to be reinforced against the immense surface pressure from its thick clouds. The moon and Mars - past and future destinations for astronauts - are both possible for astronauts to walk on, however, though those gravitational fields would take some getting used to.

A new paper in the journal Astrobiology (also available in preprint version on Arxiv) says the "Star Wars" problem is understandable because "filming is done on Earth, and an accurate representation of other gravity fields would be technically difficult and expensive." But is it representative of exoplanets generally? Led by the University of Valencia's Fernando Ballesteros, the authors say reality may actually be not too far off.

The authors say more than 2,000 extrasolar worlds (both confirmed and plausible) have been found, at a rate of about three per week since 2011. That's mostly thanks to NASA's prolific Kepler space telescope. While Kepler finds planets by looking at the dip in light they produce when passing across a star, other telescopes measure the gravitational wobble these planets create in the star instead. That wobble gives an estimate of how massive the planet is.

The authors classify found exoplanets into three categories: 1) masses below Earth (like Mars), 2) a transition zone with super-Earths, Neptunes and some solar system planets, 3) gas giants with masses hundreds of times that of Earth. Surprisingly, that "transition zone" has several planet analogs in our own solar system with surface gravities similar to Earth: Venus, Uranus, Neptune, and Saturn. (Note again that the gravity of Venus is similar to Earth, but its atmosphere can quickly crush unprotected spacecraft.)

But it's still unclear what the precise relationship is between planetary masses and their diameters. "For a given mass one could expect a diversity of sizes depending on the planetary composition and atmospheric size," the authors write, "and we do not even know whether all that we call super-Earths have a solid surface."

Artist's impression of an astronaut on Mars. Conveniently, Mars has a friendly surface gravity for Earth-arriving astronauts, but most planets in the solar system do not. |
NASA

The authors add that in theory, you could have a huge rocky planet with no natural atmosphere, but that is challenged by current planetary formation models. Generally it is believed that planets assembled with bits of rock and gas attracting each other over time.

"One could in principle propose a rocky planet as big and massive as one would wish, with no atmosphere at all, but no natural process produces it. The accretion process and the competition for materials during planetary formation impose severe constraints on feasible planets," the authors wrote.

But the authors note that several super-Earths that are both rocky, and that have surface gravities similar to our own planet, have already been spotted by telescopes. So perhaps the "Star Wars" strolls are not too far-fetched, they said.

"If while viewing ‘The Force Awakens" the reader sees Harrison Ford walking on Takodana as if he were strolling down Hollywood Boulevard, do not be too critical," they said at the end of the paper. "After all, this may not be so wrong."

, a "dynamic journey behind the scenes of the next step in the evolution of telescopes: NASA's James Webb Space Telescope," directed by Oscar®-nominated filmmaker Nathaniel Kahn. This next-generation telescope will be 100 times more powerful than the NASA/ESA Hubble Space Telescope, but Hubble, which has been observing the cosmos for quarter of a century, continues to dazzle us with stunning views of the universe.

Hubble is famed for the deep views of the cosmos it is able to generate, seeing some of the earliest galaxies that formed billions of years ago. But with the help of a spacetime quirk, as predicted by Albert Einstein 100 years ago, Hubble is now using gravitational lensing as a means to superboost its vision even further. As part of the Frontier Fields project, this observation revealed hundreds of baby galaxies never before seen, from just 600 million years after the Big Bang.

Galaxies are certainly Hubble's "thing" and these last 12 months have been no different. Seen here is one such galaxy, but it can't be easily categorized; it's neither elliptical or spiral, it is "irregular." Though they don't seem to conform to specific formation rules, irregular galaxies are still beautiful, as this example shows.

Not to be left out on 2015's feverish buildup to "Star Wars Episode VII: The Force Awakens" in December, Hubble released this very sci-fi view of a Herbig-Haro (HH) object -- basically a violent young star. This particular example seems to be channeling its own Star Wars spirit, producing a double-bladed lightsabre.

First observed in 2014, this ancient supernova has popped up three more times, in different locations in a galactic cluster. This quirk of spacetime -- gravitational lensing -- can cause beams of light to travel different paths around massive objects. Sometimes, as Hubble has shown, although they all originate from the same source, 4 points of light can appear as a cross (nicknamed an "Einstein Cross").

Not forgetting the objects in our own backyard, Hubble was commanded to spend 10 hours looking at the biggest planet in our solar system, revealing previously unseen detail in the gas giant's beautiful atmosphere. Watch Jupiter's Big Red Spot rage in one of the most stunning 2-frame animations of the year.

Ready for some hard core physics? Though Hubble can't technically "see" black holes themselves, it can certainly see their impact on local space. And in this case, the space telescope saw relativistic shocks in the flow of material blasting from a supermassive black hole 260 million light-years from Earth.

In new images released by Hubble, some stunning detail in the nebulous aftermath of a supernova has been revealed. Shown here is the cooling plasma in the Veil Nebula, the remnant of a star that exploded 8,000 years ago.

In April 2015, Hubble officially turned 25 years old and to celebrate this view of the star-forming region in the cluster Westerlund 2 in the Gum 29 interstellar cloud nebula was released. In the observation is a 2 million year-old cluster of around 3,000 stars that are in the process of being born.

Like all good parties, there's going to be some damage. And this is certainly the case for the young bright stars inside the open cluster
Trumpler 14 where a huge void is being blasted into a star-forming nebula by the baby stars it once nurtured.

You may remember Hubble's early observations of the "Pillars Of Creation" that featured giant columns of dust and gas, beaded with young stars in their stellar cocoons. Now Hubble has revisited the Pillars to reveal a new 3-D view showing us a lot more spatial detail in this famous photo.

Hubble has shown itself to be very adept at discovering planets orbiting other stars and the stellar disks they are birthed from. In these new observations of the young star Beta Pictoris' protoplanetary disk, a strange stirring was detected in the disk's dust -- revealing the presence of an otherwise invisible giant exoplanet.

Trapped in a vast, empty region of the universe this galaxy seems to be the loneliest galaxy in the universe. Voids are known to exist throughout the cosmos, flanked by a "web" of dark matter and galaxies, but they are not

completely

empty. Often, as this elegant spiral galaxy shows, entire galaxies can become marooned and there is little idea as to whether they were born there, alone, or flung there billions of years after some massive gravitational upheaval.